If someone can confirm LNAV/VNAV works with any number of inop engines, I'll remove that rule from PSX.

Britjet wrote

Not sure myself. I seem to remember seeing this is the big sim as well once, but can't be sure. Sorry. There appears to be no tech information on this at all. Strange one...Yet if this was the case you would think it would be in the Boeing QRH...

It seems the QRH doesn't always tell every detail. E.g. for all the probe heat failures it doesn't say that LNAV/VNAV will be inop. Other documents and at least one real-world incident showed that LNAV/VNAV is unavailable when any probe heat power is off.

I think the FMC uses the same "engine-running" condition that is included in the validation of the A/T engagement. And that is, if I recall correctly, N2/N3 > 45%, and the FMC gets the N2/N3 data from the EIU.

The FMC decides whether LNAV/VNAV is to be engaged. It can be armed anytime. But the engagement criteria are computed in the FMC.